The present invention relates to a cooling system for cooling an engine by circulating an engine oil and, more particularly, to a cooling system for cooling a four-cycle engine utilizing an oil tank device having an improved air bleeding structure.
In a conventional four-cycle engine of a motorcycle, for example, an engine oil is circulated through an oil tank device as a cooling medium for cooling the engine as well as lubricating the same. In the conventional engine cooling system, the engine oil is forciblY supplied to a cylinder head of the engine to cool the cylinder head at which the engine is most highly heated, and the engine oil after the utilization for cooling the cylinder head is returned to an oil pan in a crank case or a clutch chamber through a return pipe or return passage in a cylinder block.
Usually, in the conventional engine cooling system described above, there is a fear of agitating the engine oil returned to the oil pan because of the arrangement of a number of rotating members such as gears and crank shaft and, hence, increasing the temperature of the engine oil, resulting in the degradation of the engine oil. Furthermore, in this connection, there is also a fear that the rotating members will agitate a voluminous amount of the engine oil in the oil pan, resulting in the mechanical loss of the rotating members.
In a conventional motorcycle, the engine cooling system utilizing an oil tank device is composed by combining a hollow main tube, a hollow downtube, a hollow tension tube, and the like for circulating the engine oil, for example, as described in the Japanese Patent Publication Nos. 5130/1977 and 5131/1977. In another type of conventional motorcycle, there is provided a frame member having an inner hollow portion which is utilized as an oil tank.
However, with these oil tank devices, voluminous air is contained in the engine oil introduced from the oil pump into the oil tank. The engine oil together with the air is again circulated in the oil pan of the engine, resulting in the problem of inadequate engine lubrication and hence degrading the cooling function of the engine. No positive means for bleeding the air from the engine oil has heretofore been provided for the oil tank device of the type described above. In order to obviate these defects, there is also provided an oil tank device of a motorcycle in which the engine oil including the air is circulated through a long passage formed by a pipe structure of the frame of the motorcycle and the air is bled during the circulation.
With the construction of the oil tank described above, however, it is impossible to completely eliminate or bleed the air from the engine oil. The inclusion of the air in the engine oil may cause the mechanical loss of components of the engine unit such as the crank shaft, generate noise and make instable the pressure, the temperature and the circulation of the engine oil, which may finally result in an over-heat condition, the seizing of the engine and the lowering of the durability of the engine components as well as the degradation of the engine cooling function.